Two-time Wimbledon champion Petra Kvitova says some players will not go to the US Open if current coronavirus restrictions continue.
The US Open is due to be played without fans in New York from 31 August.
The country is still reporting tens of thousands of new coronavirus cases daily and possible quarantine rules for players are unclear.
"I know a few players will definitely not go if the restrictions are like they are now," Kvitova told BBC 5 Live.
The 30-year-old Czech added: "I'm still thinking of what everything will look like, what the restrictions will be, how many people we can take and if they quarantine us."
World number one Novak Djokovic has previously said he is "not sure" whether he will play, while defending men's champion Rafael Nadal has also expressed reservations.
The US has reported more than 3.4 million cases of coronavirus, and more than 136,000 deaths nationwide, according to Johns Hopkins University.
Kvitova - who is ranked 12th in the world - is one of several top players competing in front of fans at the Bett1 Aces exhibition tournament in Berlin.
It is the first sporting event in Germany to take place with spectators, who have to socially distance in the stands.
Kvitova has played behind closed doors in the past and describes the experience as "weird", adding that she cannot envisage playing a major tournament in those conditions.
"Playing without the fans in Grand Slams, I can't really see it," she said.
"If that happens and everything is OK I will go for sure to compete but there's still a chance I will not go. I will decide when I know everything."
Meanwhile, 23-time Grand Slam champion Serena Williams will return to the court for the first time since before the start of the pandemic at the inaugural edition of the Top Seed Open in Lexington, Kentucky, which starts on 10 August.
Fellow American and former US Open winner Sloane Stephens will also play in the tournament.
It is one of two events - the other being in Prague, Czech Republic - to be added to the calendar of the WTA Tour, which will resume in Palermo, Italy on 3 August.
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